1. Field of the Invention
This invention relates to the field of analog CMOS integrated circuit design, and specifically to integrated operational amplifiers driving low impedance loads.
2. Prior Art
A fully differential output operational amplifier is normally superior to a single ended output operational amplifier in terms of power supply noise rejection and noise coupling rejection, since the power supply variation or any noise source couples equally to both output branches at the same time and thus appears as a common mode signal not effecting the differential output of the operational amplifier. This is especially important for a mixed mode chip (integrated circuit having both analog and digital circuits on the same substrate), where the noise is generated from the switching of the digital circuits.
In the prior art, fully differential output CMOS power amplifiers are implemented using typically a two (or sometimes more) stage operational amplifier which are powered from the normal system power supply VCC. The typical two stage operational amplifier consists of a mostly differential voltage amplifying input stage for noise and offset considerations, and a current gain and/or MOS output driver stage for driving a low impedance load. These two stages both get their power from VCC. Such CMOS operational amplifiers are difficult to operate from a low voltage power supply for driving low impedance loads, such as an 8 ohm speaker, since the gate driving ability of the MOS output stage is so limited. The limited gate drive could be made up in substantial part by increasing the size of the output devices, but this could make the MOS driver prohibitively large.
Conventional enhancement mode n-MOS input stages for low voltage operation also suffer from minimum input common mode range, since the input voltage has to be at least larger than the VT (threshold voltage) of the input n-MOS. Also, conventional enhancement mode n-MOS source follower output drivers suffer from a limited swing on the output voltage in the positive direction because of the VT drop from the gate to source.